A disk drive is a digital data storage device that stores information on concentric tracks on a storage disk. The storage disk is coated on one or both of its primary surfaces with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a constant rate. To read data from or write data to the disk, a magnetic transducer (or head) is positioned adjacent to a desired track of the disk while the disk is spinning.
Writing is performed by delivering a polarity-switching write current signal to the magnetic transducer while the transducer is positioned adjacent to the desired track. The write signal creates a variable magnetic field at a gap portion of the magnetic transducer that induces magnetically polarized transitions on the desired track. The magnetically polarized transitions are representative of the data being stored.
Reading is performed by sensing the magnetically polarized transitions on a track with the magnetic transducer. As the disk spins adjacent to the transducer, the magnetically polarized transitions on the track induce a varying magnetic field into the transducer. The transducer converts the varying magnetic field into a read signal that is delivered to a preamplifier and then to a read channel for appropriate processing. The read channel converts the read signal into a digital signal that is processed and then provided by a controller to a host computer system.
When data is to be written to or read from the disk, the transducer must be moved radially relative to the disk. In a seek mode, the transducer is moved radially inwardly or outwardly to arrange the transducer above a desired track. In an on-track mode, the transducer reads data from or writes data to the desired track. The tracks are typically not completely circular. Accordingly, in the on-track mode the transducer must be moved radially inwardly and outwardly to ensure that the transducer is in a proper position relative to the desired track. The movement of the transducer in on-track mode is referred to as track following.
Modern hard disk drives employ a dual-actuator system for moving the transducer radially relative to the disk. A first stage of a dual-actuator system is optimized for moving the transducer relatively large distances. A second stage of a dual-actuator system is optimized for moving the transducer relatively small distances. The present invention relates to hard disk drives having dual-stage actuator systems.
FIGS. 1 and 2 depict a mechanical portion of an example disk drive 10. The disk drive 10 further comprises control electronics typically including a preamplifier, a read/write channel, a servo control unit, a random access memory (RAM), and read only memory (ROM), spindle motor, and dual-stage driving electronics. The electronic portion is or may be conventional and will not be described herein beyond what is necessary for a complete understanding of the present invention.
FIGS. 1 and 2 show that the mechanical portion of the disk drive 10 includes a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. The disk drive 10 includes at least one and typically a plurality of disks 12, each with one or two recording surfaces. During use, the disk 12 is rotated about a spindle axis A. The term “cylinder” is often used to refer to the tracks on each of the recording surfaces that are located at the same radial distance from the spindle axis.
The disk drive 10 further comprises what is commonly referred to as a head 18. The head 18 comprises or supports the magnetic read/write transducer described above; the head 18 will be referred to herein as the component of the disk drive 10 that reads data from and writes data to the disk 12.
FIGS. 1 and 2 further illustrate a positioning system 20 of the disk drive 10. The positioning system 20 comprises a bearing assembly 22 that supports at least one actuator arm assembly 24. The actuator arm assembly 24 supports the head 18 adjacent to one recording surface 26 of one of the disks 12. Typically, the bearing assembly 22 will support one actuator arm assembly 24 and associated head 18 adjacent to each of the recording surfaces 26 of each of the disks 12. The actuator arm assemblies 24 allow each head 18 to be moved as necessary to seek to a desired track 27 in seek mode and then follow the desired track 27 in track following mode.
Typically, the actuator arm assemblies 24 are fixed relative to each other; the positioning system 20 thus moves at least a portion of the actuator arm assemblies 24 together. In this case, all of the actuator arm assemblies 24 will be located adjacent to the same track on each of the recording surfaces, or, stated alternatively, at the same cylinder.
The positioning system 20 depicted in FIGS. 1 and 2 is a dual-stage system. Accordingly, each actuator arm assembly 24 comprises a first actuator structure 30 and a second actuator structure 32. For ease of illustration, FIGS. 1 and 2 depict the first and second actuator structures 30 and 32 as comprising first and second elongate actuator arms 34 and 36, respectively, and the actuator structures 30 and 32 may be implemented as shown in FIGS. 1 and 2.
The actuator structures 30 and 32 may, however, be implemented using other structures or combinations of structures. For example, the first actuator structure 30 may comprise an elongate arm that rotates about a first axis B, while the second actuator structure 32 may comprise a suspension assembly rigidly connected to a distal end of the first actuator. In this case, the first actuator is able to rotate about an actuator axis, while the head 18 would be suspended from the second actuator for linear movement along the disk radius relative to the position of the first actuator. The actuator structures 30 and 32 may thus take any number of physical forms, and the scope of the present invention should not be limited to the exemplary actuator structures 30 and 32 depicted in FIGS. 1 and 2.
Conventionally, the bearing assembly 22 is also considered part of the first actuator structure 30. In particular, the bearing assembly 22 supports a proximal end 40 of the first actuator arm 34 for rotation about a first axis B, while a distal end 42 of the first actuator arm 34 supports a proximal end 44 of the second actuator arm 36 for rotation about a second axis C. In this case, the head 18 is supported on a distal end 46 of the second actuator arm 36.
FIG. 2 also illustrates that the exemplary actuator structures 30 and 32 of the positioning system 20 form part of a first actuator 50 and a second actuator 52. For the purposes of the following discussion, the first actuator 50 is identified as a voice coil motor (VCM) and the second actuator 52 is identified as a piezoelectric transducer (PZT). However, the actuators 50 and 52 may be formed by any device capable of movement in response to an electrical control signal as will be described below.
In particular, based on a first actuator control signal, the first actuator 50 moves the first actuator arm 34 to change an angular position of the head 18 relative to the first axis B. The second actuator 52 is supported by the distal end 42 of the first actuator structure 30 to rotate the head 18 about the second axis C based on a second actuator control signal. In FIG. 2, an angular position of the first actuator arm 34 is represented by reference character D, while an angular position of the second actuator arm 36 is represented by reference character E.
A range of movement “S” associated with the second actuator structure 32 is defined by the stroke “s+” and “s−” in either direction relative to a neutral position D defined by the first actuator arm 34. The term “actual displacement” (ds in FIG. 2) refers to the angular difference at any point in time of the head 18 relative to the neutral position as defined by the position D of the first actuator structure 30. When the head 18 is in the neutral position, the actual displacement of the second actuator arm 36 is zero.
FIG. 2 further identifies arbitrary first and second tracks TA and TB on the disk 12. The actuator arm assembly 24 is shown in an initial position by solid lines and in a target position by broken lines; the first track TA will thus be referred to as the “initial track” and the second track TB will be referred to as the “target track”. It should be understood that the terms “initial track” and “target track” are relative to the position of the head 18 before and after a seek operation. Any track 27 on the disk 12 may be considered the initial track or the target track depending upon the state of the disk drive 10 before and after a particular seek operation.
FIG. 3 contains a block diagram of a servo system 60 incorporating a conventional two-stage actuator system. The servo system 60 will typically be embodied as a software program running on a digital signal processor, but one of ordinary skill in the art will recognize that control systems such as the servo system 60 described herein could be implemented in hardware.
The servo system 60 comprises a first stage 62 and a second stage 64. As described above, the disk 12 defines a plurality of tracks 27 in the form of generally concentric circles centered about a spindle axis C. The first stage 62 controls the VCM 50 and the second stage 64 controls the PZT 52 to support the head 18 adjacent to a desired one of the tracks 27. The first and second actuator control signals are generated as part of this larger servo system 60.
More specifically, an input signal “R” is combined with a position error signal “PES” by a first summer 70. The second stage position signal Y2 is indicative of an actual position of the actuator 52 of the second stage 64, and a second stage position estimate signal “Y2est” is indicative of an estimated position of the actuator 52 of the second stage 64. The second summer 72 combines the second stage position estimate signal “Y2est” and the output of the first summer 70. A first stage position signal “Y1” is indicative of the actual position of the first actuator 50 of the first stage 62. A third summer 74 combines the first and second stage position signals “Y1” and “Y2”. System disturbances “d” are represented as an input to the third summer 74. The position error signal “PES” thus represents the combination of the first and second position signals “Y1” and “Y2” with any system disturbances “d”.
The source of the input signal “R” and the first and second stage position signals “Y1” and “Y2” is or may be conventional and will be described herein only to the extent necessary for a complete understanding of the present invention. As will be described in further detail below, each of the tracks T contains data sectors containing stored data and servo sectors containing servo data. The servo data identifies each individual track T to assist in seek operations and is also configured to allow adjustment of the radial position of the head 18 during track following. As is conventional, a servo demodulation unit generates the position error signal “PES” and the first and second stage position signals “Y1” and “Y2” based on the servo data read from the disk 12. The input signal “R” is generated by a host computer or is simply zero during track following.
Referring now back to the servo system 60, the overall bandwidth of the system 60 is determined by the second stage 64. The gain variation of the second stage 64 thus directly affects the bandwidth and stability margins of the entire system 60. The need thus exists to calibrate the gain of the second stage 64 to improve drive performance (consistent system bandwidth) and reliability (consistent stability margins, accurate screening during the self-test).
However, PES non-linearities can adversely affect conventional methods of calibrating the gain of the second stage 64. In particular, a conventional method of measuring the gain of the second stage 64 is to operate the first stage 62 to perform a track follow operation on a particular track while applying a calibration signal of known amplitude the second stage 64. The calibration signal is predetermined to cause movement of the second actuator arm 36 relative to the position D of the first actuator arm 34 equal to approximately one-half the width of the track. By monitoring the servo data read by the head 18 while the calibration signal is applied to the second stage 64, the actual displacement of the second actuator arm 36 relative to the position D can be measured.
The relationship between the calibration signal, the PES signal, and the actual displacement of the second actuator structure 32 during calibration thus allows a calibration factor to be generated for a particular positioning system. The PES signal is thus an important factor when calibrating the second stage 64, and PES non-linearities adversely affect the ability of the positioning system to generate an accurate calibration factor.
The inventors have recognized that one important cause of PES non-linearities is the configuration of the servo sectors formed on the recording surfaces. In particular, referring now to FIG. 4, depicted therein is a somewhat simplified schematic representation of three adjacent tracks TX, TY, and TZ. The track TX is the outermost track on the recording surface in the example depicted in FIG. 4. The track TY is radially adjacent to the track TX but is located inwardly of the track TX, while the track TZ is radially adjacent to the track TY but is located inwardly thereof.
As shown in FIG. 4, a plurality of data sectors 120 is associated with each of the tracks TX, TY, and TZ, and between each of the data sectors 120 is a servo sector 122. As generally discussed above, the servo sectors 122 contain the servo data that allows the position of the head 18 to be determined for seek operations and accurate track following.
More specifically, each of the example servo sectors 122 comprises a plurality of A bursts 130, B bursts 132, C bursts 134, and D bursts 136. The bursts 130-136 are formed in radial sequences 140, 142, 144, and 146, with one such radial sequence 140-146 of each of the servo bursts 130-136 associated with each of the tracks T. Each of the radial sequences 140-146 of servo bursts 130-136 may, however, be associated with more than one of the tracks T.
An A/B burst seam 150 is defined between each sequence 140 of A bursts 130 and the adjacent sequence 142 of B bursts 132. The A/B burst seams 150 extend along the tracks T and, ideally, define the centers of the tracks T. A C/D burst seam 152 is similarly defined between each sequence 144 of C bursts 134 and the adjacent sequence 146 of D bursts 136. The C/D burst seams 152 are typically offset from the A/B burst seams. The burst seams 150 and 152 ideally extend in a direction parallel to the tracks TX, TY, and TZ.
In addition, burst transitions are formed between the leading and trailing edges of circumferentially adjacent bursts. The example in FIG. 4 includes an A/B burst transition 154 formed between the trailing edge of each A burst 130 and the leading edge of the B burst 132 adjacent thereto. Similarly, a B/C burst transition 156 is formed between the trailing edge of each B burst 132 and the leading edge of each C burst 134 adjacent thereto. In the example shown in FIG. 4, a C/D burst transition 158 is also formed between the trailing edge of each C burst 134 and the leading edge of each D burst 136 adjacent thereto. The burst transitions 154-158 ideally extend in a direction perpendicular to the tracks TX, TY, and TZ.
FIG. 4 is only one example of a configuration of a servo sector. Servo sectors may contain a lesser or greater number of servo bursts than the four servo bursts depicted in FIG. 4. In addition, the servo bursts may be formed in different patterns. However, burst seams that extend parallel to the track direction and burst transitions that extend perpendicular to the track direction will typically be defined between adjacent servo bursts.
It should be noted that FIG. 4 is highly idealized in that the various bursts are perfectly aligned with each of the burst seams 150 and 152 and burst transitions 154-158. In practice, servo bursts are often not perfectly aligned with the burst seams 150 and 152 and/or the burst transitions 154-158. For example, FIG. 5 depicts a situation in which the trailing edges of the A bursts 130 associated with the track TX overlap the leading edges of the B bursts 132 associated with that track TY at the A/B burst transitions 154. The trailing edges of the A bursts associated with the track TX, on the other hand, are spaced from the leading edges of the B bursts associated with that track TZ at the A/B burst transitions 154.
Although FIG. 5 depicts a simplified example in which problems occur only at the A/B burst transitions 154, similar problems may occur at the B/C burst transitions 156, at the C/D burst transitions 158, at the A/B burst seams 150, and at the C/D burst seams 152. Burst misalignment such as is depicted in FIG. 5 will be referred to herein as burst alignment anomalies.
As is well-known in the art, the servo bursts are formed by specialized factory formatting equipment during what is referred to as a low-level disk format process. In particular, before the low-level disk format process is performed, the disk is blank and contains no information of any kind. To allow the hard disk drive to be used, the factory formatting equipment initially writes servo bursts to the blank disk surface.
As one example of the low-level format process, the outermost radial sequence of servo bursts associated with the outermost track (e.g., sequence 140 of D bursts 136 associated with the first track TX in FIG. 4) may first be written to the disk. If the low-level format process starts at the outermost edge of the recording surface, the servo bursts located radially inwardly of the outermost servo bursts (e.g., sequence 142 of A bursts 130 associated with the track TX in FIG. 4) are next written to the disk in a second radial sequence. This process is repeated by forming successive radial sequences of servo bursts, while moving towards the spindle, until all of the radial sequences of servo bursts have been written.
Burst alignment anomalies may be, and often are, created during the low-level disk format process. The inventors have further recognized that, because of the sequential nature in which servo bursts are written during the low-level disk format process, burst alignment anomalies tend to be consistent along burst seams and/or burst transitions between adjacent radial burst sequences and tend to vary from one burst seam and/or burst transition to another radially-spaced burst seam and/or transition.
In particular, referring for a moment back to FIG. 5, in that example, the spacing between the trailing edges of the A bursts 130 and the leading edges of the B bursts 132 associated with the first track TX is consistent along the track TX. Similarly, the spacing between the trailing edges of the A bursts 130 and the leading edges of the B bursts 132 associated with the second track TY is consistent along that track TY. However, a comparison of the relative locations of the A bursts and B bursts of the first and second tracks TX and TY illustrates that the burst alignment anomalies associated with these tracks TX and TY differ.
In the context of calibrating the gain of the second stage 64, the inventors have further recognized that, because burst alignment anomalies tend to be consistent along burst seams and/or at burst transitions at adjacent radial burst sequences, the non-linearities in the PES will be consistent for a particular track. Accordingly, if the second stage is calibrated while following a particular track, the consistency of the burst alignment anomalies in that particular track creates a consistent, unknown non-linearity in the PES that cannot be eliminated by averaging, prediction, or other post-processing of the PES signal.
A need thus exists for improved positioning systems and methods for a dual-stage actuator of a disk drive and, in particular, for improved calibration systems and methods for the second stage of such positioning systems and methods.